Supersonic wind tunnel



Oct. 2, 1951 R H JOHNSON 2,570,129

SUPERSONIC WIND TUNNEL Filed Aug. 18, 1948 Inv@ OT`:

Robert; H, hhSOn,

by @awe/. 1

Patented ct. f2, 195i s'UPEItsoNIc WIND TUNNEL Robert H. Johnson,Schenectady, N. Y., assignoI to General Electric Company, a corporationof New York Application August 1s, 194s, serial No. 44,9%

solaims. (c1. z3- 147) My' invention relates to supersonic wind tunnelsand more particularly to a supersonic wind tunnel in which the startinglosses are relatively low and to the special method employed inobtaining low starting losses.

In supersonic wind tunnels at the point of minimum area, the throat ofthe flow channel, the Mach number is theoretically equal to 1. As isknown in the art, the Mach number that is attained at any point in theflow channel downstream of the throat is a function of the ratio of thearea of the channel at that point to the area of the throat. Anincreasing area ratio downstream of the throat is accompanied by anincreasing Mach number and as the Mach number increases the staticpressure decreases. The supersonic state is an unstable state and thereis a tendency of the entire now system to return to the subsonic statewhich entails an increase in the entropy level accompanied byinstantaneous variations in the pressure, temperature, and velocity ofthe fluid. This change from a supersonic state of flow to a subsonicstate of flow occurs almost instantaneously and is known as a shock. Thehigher the Mach number at which the shock occurs, the greater are thelosses in the system. Therefore, it is very desirable to have thetransformation from supersonic to subsonic ow occur at a low shock Machnumber. However, since the Mach number is a function only of the arearatio, the most eflicient shock occurs at a point at which the Machnumber is slightly greater than 1 and this point is one at which thearea is slightly greater than the throat area.

The ordinary supersonic wind tunnels have an expanding section after thethroat of the wind tunnel. In an effort to cut operating losses,supersonic wind tunnels have been built with a throat, an expandingsection, a test section, a

contracting section down-stream of the tunnel test section, and thendown-stream of the contracting section is located a second throat whichis to some extent of greater area than the rst throat. Inasmuch as theentropy level of the fluid in the tunnel has increased between the firstand second throat due to frictional losses, boundary layer losses,losses due to the drag of the model being tested, and other minor losseswhich may occur between the iirst and second throats, the'second throatmust of necessity be greater than the rst throat so that choking doesnot occur in the second throat.

In operating a standard supersonic wind tunnel in which there are twothroats and a test `section between the two throats, after choking isestablished in the met throat at which time' the Mach number is 1 at thefirst throat, as the pressure differential across the wind tunnel isincreased the supersonic flow starts down the expanding section of thewind tunnel, the downstream boundary of said supersonic flow being ashock wave. As the pressure differential is increased the shock wavemoves through the expanding section into the test section then throughthe test section into the contracting section whereupon the shock jumpsthrough the second throat to some point beyond the second throatcompatible with the existing pressure diierential. At that timesupersonic flow fills the entire channel between the first and secondthroats and up to the shock wave. 'Ihen by reducing the pressuredilerential the shock can be made to back up to as close to the secondthroat as possible and in that way the operating losses are reducedbecause the shock Mach number is reduced. Because of the diflicultyentailed in pulling the shock through` the second throat it has been thepractice to make the second throat very much greater than the iirstthroat and once the shock is pulled through the second throat, then bysome mechanical means the second throat is closed down to a point whichstill permits the total mass flow to pass and then the shock isbacked-up close to the second throat. In that manner losses are reducedby lowering the shock Mach number. These methods take care of theproblem of reducing the operating power losses.

The question of economic starting of the supersonic wind tunnel stillposes an unsolved problem. In the known methods of operation, since themoving shock must traverse the greatest area between the first andsecond throats it is necessary that suilicient power be supplied toobtain a pressure diierential that corresponds to the losses across theshock when the maximum Mach number is obtained lbetween the nrst andsecond throats. Thus, the power systems of supersonic wind tunnels aredesigned to supply sufficient power at the above critical point whichpoint is a -function of the maximum Mach number obtained in the tunnel.But once supersonic now is established between the two throats, thepower requirements are only those necessary to maintain the mass flowthrough the tunnel and to satisfy the pressure differentialcorresponding to the shock Mach number which is `beyond the secondthroat. At present, the power requirements necessary to maintain massilow are much smaller than those required for starting the supersonictunnel.

If the necessity of the movement of the shock Vsubsonic type diifuser 8.

wave through the entire range of Mach numbers occurring between thefirst and second throats can be eliminated, then the starting powerrequirements of the supersonic wind tunnel c-an be greatly reduced andthe power requirements needed for operation of such a wind tunnel wouldonly be those power requirements necessary to maintain mass flow throughthe second tunnel to a point beyond the second throat.

Thus, one of the objects of my invention is to provide a new type ofsupersonic wind tunnel.

Another object of my invention is` to provide a supersonic wind tunnelwhich has lower starting power requirements. w

Another object of my invention is to provide a supersonic Wind tunnelthat is easily constructed and is readily adaptable to ordinary sourcesof power and whose power requirements are relatively low and comparableto the power requirements for the maintenance of mass flow through thetunnel.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty In thetwo-dimensional tunnel, the area at any l section isa function onlyofthe ordinate of the section. I In Fig. l, the tunnel is shown as it willappear during the starting of the tunnel.

Fig. 2 is a cross section view of the tunnel at the plane 2,--2 Vshownin Fig. l. l

Fig. 3 shows thesupersonic tunnel Yas it will appear during,l testsafter 1 supersonic flow has been, established throughout. s

In Fig. l, Ihave shown a supersonic wind tunnel nozzle contour block Iwhich ,is shaped on one side so as to form one wall of thetwo-,dimensional supersonic channel. The Various sections of the contourare an inlet subsonic nozzle 2, a first throat 3, a `supi-:frsonicexpansion contour fi, a

vparallel wall test section region 5, a supersonic compression contour,a second throat '1,v and a The opposite mating nozzle contour block 9is, a. mirror image of the one just described and the remaining twowalls are parallel iiat plates I which are fastened to and keep inalignment contour blocks I and 9.

There are further provided two exiblemetal strips I I which are of thesame width as the flow channel and which are so fastened to the contourblocks as to t the contour of the subsonic nozzle inlet 2 up to thefirst Ythroat 3. Then the metal strips extend at substantially aconstant slope until they meet the contour blocks at substantially thesecond throat 'I and from then on the metal strips follow the contour ofthe: diffuser 8. From the second throat'on down-stream, the strips IIare held in `place along the contour of the nozzle blocks by members I2in .such a manner that the strips I I mayV slide along the contourseither upstream or down-stream as the occasiondemands.'

These members I2 are fastened to the parallel walls of the tunnelandproject into the airstream. They, together wit-h'the nozzle blocks,form slots through which pass flexible -metal strips II.

'Members I2 serveY toykeep stripsI'Ifagainst Ythe 4 surface of contourblocks I and 9 but do not restrict the movement of strips I I in anup-stream or down-stream direction.

In the nozzle contour blocks I and 9, at predetermined positions as willbe hereinafter described, are a plurality of slotted holes I3, I4, I

and l through which extend a plurality of actuating rods or cables Il,I8, I and which are fastened to tabs 2I, 22, 23 and 24 welded to thebaci: side of metal strips II. By attaching the :tabs 2I, 22, 23 and 24to the strip II at certain predetermined points and by exerting forcesupon the actuating rods or cables in certain directions, as will Vbehereinafter described, the flexible walls may 4be pulled out of theposition shown in Fig. 1, and be made to assume the contours of the`nozzle blocks, as shown in Fig. 3. The nozzle blocks may be so shapedthat when the metal strips are in position against the contour blocks,the inside contour and ordinates of the metal strip satisfy the vdesignconfigurations and ordinates of a theoretically shock-free V'supersonicnozzle. By providing means to permit Lthe metal stripsto slide alongthefcontour of the block from the secondthroat on down-stream,resistance to the deformation, which would otherwise be brought about bythe forcing of the metal strips to conform to the contour of the blocks,is prevented.

It has been found that a shockless supersonic nozzle must conform to acertain shape. The supersonic expansion region 'may be of two types. Inone, the supersonic expansion region is made up of two curves, the rst,immediately' downstream of the throat, is convex and ends at "a buttheyare separated by a di'verging straightwalled portion in which sourceflow Aprevails.`

In either of the two f types, 4it is essential that the` force appliedto deform theexible wall 'be applied at the terminus of the convexcurve, which in the first case is at Athe inilection point, and in thesecond case is at the-terminalsof'the straight source flow section. Theforce should be a tension forceapplied perpendicular to the 'wall at thepoint of application and in Ythat way the iiexible Wall is made toconform to the contour of the convex section.

In order to make the flexible wall conform to the contour of the secondor concave section, itis necessary that a force be applied in suchYamanjner that there is a tendency of the flexible wa-ll to buckle, forthrough a buck-ling configuration the flexible wall can be made to t 4aconcave surface. This buckling force must be'applied-'to the flexiblewall as if it were `a column, or-in other words in a-direction up-streamfin the tunne1 and parallel to the face of the wall;

Thus, the important lpredetermined k'points of attachment of tabs 25|,22,23 and 241are shown as follows: Y

Tab 2| is fastened 'at the inflection .fpointfof thesupersonic-e'x'pansionV section. TabV 22. is fastened tothef-pointjoining the expansion sec;- .tion-to the-'test section.Tabrs23 is attachedi to the point at the end of the test section and atthe beginning of the supersonic compression section and tab 24 is at theinflection point of the supersonic compression section. The direction ofthe forces impressed upon the rods or cables I1, IB, I9 and 20 is shownby arrows in Fig. 3. The force impressed upon members I8 and 20 areshown in component form so that it can be more clearly seen how forcesare provided to obtain the buckling of member II discussed above.

In operation, a driving system which may be either a blower type systemor an induction type system is started up and operated until a pressuredifferential is obtained at which choking occurs at the rst throat atwhich time the Mach number is equal to 1 at the throat. The flexiblestrips II at that time are in their normal uniiexed position, thusgiving a substantially straight walled expanding section down-stream ofthe first throat. Although some losses occur in the section down-streamof the rst throat due to the incorrect contour for the supersonicexpansion Which occurs after the rst throat, these losses are relativelysmall. After supersonic ow is established, the pressure differential isincreased driving the shock wave down-stream through the straight taperdiffuser formed by the iiexible strips I I. When the shock wave reachesa point beyond the second throat 1, the rods or cables I1, IB, I9 and2l) are activated in such a manner that the exible metal strip Il isforced to take up the contour of the contour block in the regions of thesupersonic expansion contour 4, the parallel wall test section 5, andthe supersonic compression contour section 6. In order that the metalstrip II may take up this contour, part of the metal strip which extendsinto diffuser 8 slides forward being held in position by strips I2 andprovides a sufficient length of strip to satisfy the increase in lengthdemanded by the contour of the contour block. Inasmuch as supersonicflow has been established between the rst and second throat, this exingof the metal wall between the two throats does not disturb the flow and,in fact, once the flexible strip assumes the contour of the contourblock, the losses in the section between the two throats is considerablyreduced over what they were when the metal strip was in the straightunexed position. Since the losses are reduced, the shock wavedown-stream of the second throat will travel further downstream in thediffuser 8. By reducing the pressure differential, the shock wave can bemade to back-up close to the second throat thus reducing the operatinglosses. It will be obvious that by this starting procedure thetremendous pressure diierentials required to move the shock through aregion of high Mach number is eliminated and only a pressuredifferential slightly greater than that necessary to maintain mass iiowthrough the channel is needed to start the supersonic tunnel. It will benoted that the direction of force appliedv to the metal strip by rod orcable I1 is in a direction substantially perpendicular to the contour ofthe contour block at that point. This is also true for the direction offorce exerted by rod or cable I9 attached to that part of the stripwithin the parallel wall test section 5. However, it will be noted thatthe direction of the forces on rods or cables I8 and 20 are differentthan on the other two members I1 and I9. The purpose is to have acomponent of the force Vtransmitted through the flexible metal stripin adirection up-stream. In this way the flexible metal strip is made t fitthe supersonic expansion contour 4 very closely for since that contouris concave it is necessary that an axial force on this strip be exertedso that it will assume the contour of the block behind it. Of course,this does not assure the adherence of the flexible metal strip to theexact contour of the supersonic compression region 6, but inasmuch asthe important region for supersonic iiow is the test section 5 whichoccurs up-stream of the supersonic compression section 6, thosedisturbances which occur, due to deviations from the theoreticalshock-free shape of section 6, do not eifect the flow which occurs inthe section 5 because in supersonic iiow a disturbance that occursdownstream of a certain point will not effect the ow that occurs11p-stream of that point.

While I have shown a particular embodiment of my invention, it will beunderstood, of course, that I do not wish to be limited thereto, sincemany modifications may be made, and I therefore contemplate by theappended claims to cover any modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A two-dimensional, two-throated supersonic wind tunnel comprisingmating nozzle blocks of preshaped contour and connection side; wallsdefining with said blocks a fluid liow pass-- ageway, a flexible wallattached to each nozzle.1 block in advance of the first throat andfollowing the contour of said nozzle block substantially' to the firstthroat in said tunnel, said flexible wall then initially runningstraight and divers-- ing at a constant slope downstream of said first.throat at least to the second throat, and means: for deforming saidstraight section of the flexible walls to conform with the contour ofsaid nozzle: blocks.

2. A two-dimensional supersonic wind tunnel, comprising nozzle blocks ofpre-shaped contour' forming, in order progressing down-stream, an. inletnozzle section, a rst throat, a supersonic; expansion section, aparallel walled test section a supersonic compression section, a secondthroat and a diffusing section, a pair of flexible walls; attached tosaid nozzle blocks and following thee contour of said nozzle blockssubstantially to the: first throat in said tunnel, the said flexiblewalls; then running straight and diverging at constant; slopedown-stream of the said first throat, andi means for deforming saidstraight sections of thea flexible walls to conform with the contour of.said nozzle blocks.

3. A two-dimensional supersonic wind tunnell comprising nozzle blocks ofpre-shaped contour: forming, in order progressing down-stream, arr inletnozzle section, a rst throat, a supersonic-y expansion section, aparallel walled test section,. a supersonic compression section, :asecond throat and a diffusing section, a pair of flexible walls:attached to said nozzle blocks and following the contour of said nozzleblocks substantially to the first throat in said tunnel, said flexiblewalls then running straight and diverging at constant slope down-streamof the rst throat substantially to the second throat, said iiexiblewalls then following the contour of said nozzle blocks down-stream ofthe second throat and being free to slide along the contour of thenozzle block in both an up-stream and down-stream direction, and meansfor deforming said straight sections of the flexible wall to conformwith the contour of said nozzle blocks.

4. A two-dimensional supersonic wind tunnel comprising nozzle blocks ofV.pre-shaped contour formingi in order progressing down-stream, .aninlet nozzle section, a first throat, a supersonic expansion section, aparallel walled. test section, a supersonic compression section,v asecond throat,and a diffusing section, 4a pair of flexible. wallsattached to said nozzle .blocks and f ollow ing the contour of saidnozzle blocks substaIi-IY tially to the rst .throat in said tunnel, saidflexible walls 'thenrunning straight and diverging at constant slopedown-stream of the first throat substantially to .the secondthroat,rsaid flexible walls thenfollowing the contourlof said nozzle blocksdown-stream of the second throat and being free to slide along thecontour of said nozzle blocksin an 11p-stream and down-stream direction,each nozzle block having a Vplurality of passageways through which passforcecarrying members attached to the corresponding flexible Wall,andvmeans comprising said force--Y carrying members for deforming saidVflexible walls to conform with the contour of said nozzle blocks.

5. A two-dimensional two-throated supersonic Wind tunnel comprisingnozzle blocks of preshaped contour, a pair of iiexible walls attached tosaidV nozzle blocks and following the contour of 4said nozzle blockssubstantially to the first throat in said tunnel, said flexible Wallsthen running straight and diverging at constant slope down-stream ofsaid first throat, each nozzle block having a plurality of passagewaysthrough which pass tension members attached to the correspondingflexible wall and means compris- 8 ing saidftensionmembersfor deformingthe e'xiaf ble walls to kconform Awith thecontour .of saidnozzle blocks.6. A twodimensional supersonic wind tunnel, comprising nozzle blocksof.pre-shaped contour' forming, in order `progressing down-stream, aninletnozzle section, a firstlthr'oat, a supersonic expansion section, aparallel walled test section, a supersonic compression section, .asecond throat, `and a diffusing section, a pairrof flexible' wallsattached to said nozzleblocks and follow-` ingthe contour of saidrnozzleblocks substantial'-, ly to the first throat in said tunnel, .saidexible walls -then running `straight and diverging `at constant slopedown-stream of the first throat substantially to the second throat, saidflexible Wallsthen following the contour of said nozzle blocksdown-streamof the second throat and being free to slide along thecontour of the nozzle blocks in an up-stream and down-stream directionand means comprising tension members for deforming the iiexible walls toconform with the contour of said nozzle blocks.

ROBERT I-I. JOHNSON;

REFERENCES CITED The following references are of recordin the le of thispatent:

UNITED STATES Y PATENTS

